The remains of a wooden ship discovered when construction workers cleared areas where the World Trade Center once stood in New York City show it was built at least 240 years ago, experts say.
Researchers used tree rings in wooden ribs recovered at the site to determine it was probably built around 1773, likely in a small shipyard in Philadelphia.
Writing in the journal Tree-Ring Research, experts said it might have been constructed from the same variety of white oak used in that city's Independence Hall, the site of the signing of the Declaration of Independence and the U.S. Constitution.
"What makes the tree-ring patterns in a certain region look very similar, in general, is climate," study leader Dario Martin-Benito said.
Wetter years create thicker rings while drier climates produce thinner ones, he said.
The researchers were able to restrict their searches to the eastern U.S. because the ship's keel was hickory, unlike the oak ribs, and hickory in the United States is confined to the east.
Comparing tree ring samples from the recovered ship timbers to live sample cores allowed the researchers to narrow down the source to Philadelphia and its immediate surroundings, he said.
"Philadelphia was one of the most -- if not the most -- important shipbuilding cities in the U.S. at the time," Martin Benito said. "And they had plenty of wood so it made lots of sense that the wood could come from there."
The wood also matched sections taken from Independence Hall, the researchers said.
Why the ship, uncovered during excavations for the World Trade Center's Vehicular Security Center, ended up more than 20 feet lower than the current street level is unclear, they said.
It may have accidentally sunk or it may have been scuttled to become part of landfill used to stabilize the coastline of Lower Manhattan, they said.
Archaeologists on-site during the excavations also recovered ceramic ware, animal bones, shoes and bottles, but the ship was the most significant find.
The recovered portion of the wreck was about 30 feet long, but the archaeologists suggest it may have been 60 to 70 feet long when still intact and afloat.
To prevent deterioration from exposure to the atmosphere, the excavated ship timbers were shipped to the Maryland Archaeological Conservation Laboratory to be kept underwater for prevent warping or cracking.
In the meantime, a few sample timbers were given to Columbia University's Lamont-Doherty Earth Observatory and its Tree Ring Laboratory, which determined the wood's origins.
